Published: February 2, 2011

CAIRO — President Hosni Mubarak struck back at his opponents on Wednesday, unleashing waves of his supporters armed with clubs, rocks, knives and firebombs in a concerted assault on thousands of antigovernment protesters in Tahrir Square calling for an end to his authoritarian rule.

Ed Ou for The New York Times

An Egyptian soldier in a truck pointed a knife at a photographer during the chaos that erupted on Wednesday in Cairo. More Photos »

Early Thursday morning, shots were fired at the anti-Mubarak protesters, a number of witnesses said. It was unclear whether the shots came from the pro-government demonstrators or from the military forces stationed in the square.

Two people were killed by the gunfire and 45 people were wounded, said a doctor at a nearby emergency clinic set up by the antigovernment demonstrators. After the gunshots, soldiers fired their weapons into the air, temporarily scattering most of the people in the square.

On Wednesday, the protesters, after first trying to respond peacefully to the Mubarak supporters, fought back with rocks and firebombs of their own. Scores of the wounded were carried back on cardboard stretchers to a makeshift clinic set up in a nearby mosque, where they were treated by dozens of doctors.

By 9 p.m. on Wednesday, government officials said, about 600 people had been wounded and three killed in the day’s battles; more than 150 people have died in the week of violence, human rights groups say. The crackdown was in defiance of calls by the United States and Europe to avoid violence, and it provoked swift condemnation and a rift with the Egyptian government, a longstanding ally.

In another sign of the shifting landscape in the Middle East, another authoritarian government made a concession to protesters as the president of Yemen, Ali Abdullah Saleh, said neither he nor his son would run for office in the next election.

The Egyptian military, with tanks and soldiers stationed around the square, neither stopped the violence on Wednesday nor attacked the protesters. Soldiers watched from behind the iron fence of the Egyptian Museum, occasionally shooting their water cannons, but only to extinguish flames ignited by the firebombs.

Only two days after the military pledged not to fire on protesters, it was unclear where the army stood. Many protesters contended that Mr. Mubarak was provoking a confrontation in order to prompt a military crackdown.

It is also possible that the military was satisfied with his decision to step down, perhaps fearful of the more radical shift to democratic elections that protesters are calling for.

Mohamed ElBaradei, who was designated to negotiate with the government on behalf of the opposition, demanded that the army move in and protect the protesters. “The army has to take a stand,” he said in a television interview. “I expect the Egyptian Army to interfere today.”

The deployment of plainclothes forces paid by Mr. Mubarak’s ruling party — men known here as baltageya — has been a hallmark of the Mubarak government, and there were many signs that the violence was carefully choreographed.

The Mubarak supporters emerged from buses. They carried the same flags and the same printed signs, and they all escalated their actions, from shouting to violence, at exactly the same moment: 2:15 p.m. The protesters showed journalists police and ruling party identification cards that they said had been taken from Mubarak supporters who had been caught infiltrating Tahrir Square, also known as Liberation Square, and detained in a holding pen.

The preparations for a confrontation began Wednesday morning, a day after Mr. Mubarak pledged to step down in September while insisting that he would die on Egyptian soil. The president’s supporters waved flags as though they were headed to a protest, but armed themselves as though they were itching for a fight. Several wore hard hats; one had a meat cleaver, and two others grabbed the raw materials to make firebombs from their car.

One man washed his pliers in a pool of dirty sewage before charging into a battle. Another man held a club wrapped in electrical tape and studded with tacks. Others carried knives, rubber tubes and chains. Before they laid siege to Tahrir Square in what seemed to be coordinated waves, they hid those weapons in their waistbands.

“He won’t go!” the Mubarak supporters chanted.

“We won’t go!” the protesters replied.

Some of the Mubarak supporters arrived in buses. When they spoke with one another, they referred to the antigovernment protesters as foreigners or traitors, and to Mr. Mubarak as Egypt’s “father.”